Based on Legacy Development with Pow from http://stuff-things.net/2011/05/16/legacy-development-with-pow/

config.ru is the key. I've installed it as described, but it couldn't find the 'rack', 'rack-legacy', 'rack-rewrite' in my rvm setup. So I added 

$:.push('/Users/johny/.rvm/gems/[email protected]/gems/rack-1.3.0/lib/')
$:.push('/Users/johny/.rvm/gems/[email protected]/gems/rack-legacy-0.2.0/lib/')
$:.push('/Users/johny/.rvm/gems/[email protected]/gems/rack-rewrite-1.0.2/lib/')

(johny being an example user as I am not Johny)

The primary code was working fine on 32-bit machine with ruby 1.9.2-p180 but when I moved to 64-bit machine with brand new rvm setup and ruby 1.9.2-p290 I had an error saying:

LocalJumpError: unexpected return
~/Documents/Dropbox/Library/code/the_app/config.ru:15
~/Documents/Dropbox/Library/code/the_app/config.ru:13:in `each'
~/Documents/Dropbox/Library/code/the_app/config.ru:13

So I modified it to not to have 'return' somewhere there and used a variable to return at the end of the `each` loop.